I'm not fond of the ':' operator since it's already used by default as inheritance operator. I would suggest something like:

[production]
phpSettings.display_errors = false
; inserts the content of subset/common.ini
; just like if it were typed directly here
@include subset/common.ini
; imports all ini files from the resources.d sub-directory
; using their name (without extension) as key
resources = @import ressources.d/*.ini

with other files in ressources.d/ looking like this:

; as it is imported from a "db".ini file, these keys
; will be treated as db.*
adapter = "pdo_mysql"
params.host = "localhost"

Your part to include multiple files by wild-cards is an interesting idea and I have the following questions/issues to this:
1. It will be only supported by ini-files or is there an xinclude port of it?

Which wild-cards should be available?
-> Only *, ?
-> Or usage of glob or fnmatch

resources = @import ressources.d/.ini
-> There is no way to create an real option value "@include foobar"
Why not using (Don't tested it yet):
resources@import = ressources.d/.ini
or resources.@import = ressources.d/*.ini

Additional: Is the a chance to import config files of different types ?
like @include subset/common.xml

It will be only supported by ini-files or is there an xinclude port of it?
To be honest I'm not a big fan of XML, and don't it that well - so I would say that if such an implementation exists, well, we can manage to support it :)

Which wild-cards should be available?
-> Only *, ?
-> Or usage of glob or fnmatch
I feel like glob would do the trick - if I don't use it, I'll be forced to use/emulate regexp, which probably wouldn't be more efficient. So glob is a good candidate for me.

resources = @import ressources.d/.ini
-> There is no way to create an real option value "@include foobar"
Why not using (Don't tested it yet):
resources@import = ressources.d/.ini
or resources.@import = ressources.d/*.ini
I didn't test neither yet, but I feel like it would be easier and more efficient not to parse the key part of the file. Using key = hard_coded_value / @import/include/something_else would be easier to handle. I wouldn't implement something to complicated if it's not to serve the usability of the component

Additional: Is the a chance to import config files of different types ?
like @include subset/common.xml
I guess it would be, but I think this would make harder to implement a caching mechanism, and overall, would be confusing to me :)

well, I've got some good news... I worked a bit on my code today, and I managed to get something working. The bad news are that it's limited to .ini files (or ini formatted files to be more accurate), and does not support Zend_Config_Writer neither.

This works with one section or more, and also with no section at all. I tested it many ways, but only manually yet. I still have to write unit tests for it. But has it mostly does... nothing but instantiating new Zend_Config_Ini objects, it shouldn't need that many tests.. Moreover, by having add a "_fileName" property, and thus creating an object for every file, it shouldn't be that difficult to add writing support...

What do you think about it (patch against latest svn version and full class file attached)?

I 'm disagreed with the way to process here. The ability to Imports a file is a good feature/idea but it sound like a big improvement and code remaniement. Also, any improvement like this must be ported in all adapters (eg. yaml, xml and so on) to be able to provide a common specification. BTW: I've your looked at the Symfony 2 to take idea for the way to process ?